1. Field of the Invention
The invention relates to a light emitting device using an organic light emitting element having an anode, a cathode, and a film (referred below to as “organic compound film”), which includes an organic compound adapted to effect luminescence upon application of an electric field. In particular, the invention relates to a light emitting device using an organic light emitting element, which is lower in drive voltage and longer in service life than a prior one. In addition, the light emitting device described in the specification of the present application indicates an image display device or a light emitting device, which use an organic light emitting element as light emitting element. Also, the light emitting device includes all of modules, in which a connector, for example, an anisotropic electro conductive film (FPC:Flexible printed circuit) or a TAB (Tape Automated Bonding) tape or a TCP (Tape Carrier Package) is mounted to an organic light emitting element, modules, in which a printed-circuit board is provided on a TAB tape or a tip end of a TCP, or modules, in which an IC (integrated circuit) is directly mounted on an organic light emitting element in the COG (Chip On Glass) system.
2. Described of the Related Art
An organic light emitting element is one adapted to effect luminescence upon application of an electric field. A mechanism for luminescence has been said to reside in that an organic compound film is interposed between electrodes, upon application of voltage thereto electrons filled from a cathode and positive holes filled from an anode recombine together at a center of luminescence in the organic compound film to form excited molecules, and the molecule excitons discharge energy to produce luminescence when returned to the base state.
In addition, kinds of molecule excitons formed by the organic compound can include a singlet excited state and a triplet excited state, while the specification of the present invention contains the case where either of the excited states contributes to luminescence.
In such organic light emitting element, an organic compound film is normally formed in a thin film below 1 μm. Also, since the organic light emitting element is a self-luminescent type one, in which the organic compound film itself emits light, a backlight used in a conventional liquid crystal display is not necessary. Accordingly, the organic light emitting element can be very advantageously formed to be thin and lightweight.
Also, with, for example, an organic compound film of about 100 to 200 nm in thickness, a time period having elapsed from filling of a carrier to recombination thereof is in the order of several tens of nanosecond taking account of the extent of movement of the carrier in the organic compound film, and luminescence is achieved in the order of less than one micro second even when the a procedure from the recombination of the carrier to luminescence is included. Accordingly, one of the features is that the speed of response is very large.
Further, since the organic light emitting element is a carrier-filling type light emitting element, it can be driven by DC voltage, and is hard to generate noise. With respect to drive voltage, an adequate luminance of 100 cd/m2 is achieved at 5.5 V by first making the thickness of an organic compound film a uniform, super-thin film of around 100 nm, selecting an electrode material, which reduces a carrier filling barrier relative to the organic compound film, and further introducing a single hetero structure (double structure) (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)).
Owing to such performances as thin and lightweight, high-speed responsibility, DC low voltage drive, and the like, organic light emitting elements have been given attention as next-generation flat panel display elements. Also, since organic light emitting elements are of self-luminescent type and large in angle of visibility, they are comparatively favorable in visibility and believed to be effective as elements used for displays in portable equipments.
Hereupon, in the constitution of an organic light emitting element described in Reference 1, a carrier filling barrier is made small relative to an organic compound film by using as a cathode a relatively stable Mg:Ag alloy of low work function to enhance an electron filling quality. This makes it possible to fill a large amount of carrier into the organic compound film.
Further, the recombination efficiency of the carrier is improved by leaps and bounds by application of a single hetero structure, in which a positive hole carrying layer composed of a diamine compound and an electron carrying luminescent layer composed of tris (8-quinolinolato) aluminum (abbreviation; Alq3) are laminated as an organic compound film, which is explained below.
In the case of, for example, an organic light emitting element having only a single Alq3 layer, a major part of electrons filled from a cathode reaches an anode without recombining with positive holes, making the luminescent efficiency very low, since Alq3 is of electron carrying quality. That is, in order to have the single layered organic light emitting element efficiently emitting light (or driving at low voltage), it is necessary to use a material (referred below to as “bipolar material”) capable of carrying both electrons and positive holes in well-balanced manner, and Alq3 does not meet such requirement.
However, application of the single hetero structure described in Reference 1 causes electrons filled from a cathode to be blocked by an interface between the positive hole carrying layer and the electron carrying luminescent layer to be enclosed in the electron carrying luminescent layer. Accordingly, the carrier is efficiently recombined in the electron carrying luminescent layer to provide for efficient luminescence.
When the concept of such carrier blocking function is developed, it becomes possible to control a carrier recombining region. As an example, there is a report, according to which success is achieved in enclosing positive holes in a positive hole carrying layer and making the positive hole carrying layer luminescent by inserting a layer (positive hole blocking layer), which is capable of blocking positive holes, between the positive hole carrying layer and an electron carrying layer (Reference 2: Yasunori KIJIMA, Nobutoshi ASAI and Shin-ichiro TAMURA, “A Blue Organic Light Emitting Diode”, Japanese Journal of Applied Physics, Vol. 38, 5274-5277 (1999)).
Also, it can be said that the organic light emitting element described in Reference 1 is based on, so to speak, that thought of functional separation, according to which carrying of positive holes is performed by the positive hole carrying layer and carrying and luminescence of electrons are performed by the electron carrying luminescent layer. Such concept of functional separation has further grown to a concept of double heterostructure (three-layered structure), according to which a luminescent layer is inserted between the positive hole carrying layer and the electron carrying layer (Reference 3: Chihaya ADACHI, Shizuo TOKITO, Tetsuo TSUTSUI and Shogo SAITO. “Eectroluminescence in Organic Films with Three-Layered Structure”, Japanese Journal of Applied Physics, Vol. 27, No. 2, L269-L271 (1988)).
Such functional separation has an advantage in that the functional separation makes it unnecessary for a kind of organic material to have a variety of functions (luminescence, carrier carrying quality, filling quality of carrier from electrode, and so on) at a time, which provides a wide freedom in molecular design or the like (for example, it is unnecessary to unreasonably search for bipolar materials). That is, a high luminous efficiency can be easily attained by combining materials having a good luminous quality and a carrier carrying quality, respectively.
Owing to these advantages, the concept of the laminated structure (carrier blocking function or functional separation) itself described in Reference 1 has been widely utilized till now.
However, an energy barrier always develops in the interface because there is contact between different types of substances in the aforementioned lamination structure. Movement of carriers in the interface is prevented if the energy barrier exists, and this raises the two problems discussed below.
First, one problem is that there arise difficulties in further reducing the driver voltage. In fact, it has been reported for current organic light emitting elements that single layer structure elements using a conjugate polymer are superior with regard to driver voltage, and that top data in power efficiency (unit of 1 m/W) is maintained (when light emission from a singlet excitation state is compared) (Reference 4: Tetsuo Tsutsui, “Applied Physics Society Organic Molecules—Bioelectronics Section,” Vol. 11, No. 1, p. 8 (2000).
Note that the conjugate polymers discussed in reference 4 are bipolar materials, and that a level of carrier recombination efficiency equivalent to that of a lamination structure can be achieved. A single layer structure with few interfaces therefore in practice shows a lower driver voltage provided that the carrier recombination efficiency can be made equivalent by a method such as using a bipolar material without the use of a lamination structure.
For example, a method exists in which a material for relieving the energy barrier is inserted in the interface with the electrode to increase the carrier injecting properties and reduce the driver voltage (Reference 5: Takeo Wakimoto, Yoshinori Fukuda, Kenichi Nagayama. Akira Yokoi, Hitoshi Nakada, and Masami Tsuchida, “Organic EL Cells Using Alkaline Metal Compounds as Electron Injection Materials”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 44, NO. 8, 1245-1248 (1997)). The driver voltage is successfully reduced by using Li2O as an electron injecting layer in reference 5.
However, carrier mobility between organic materials (for example, between a hole transporting layer and a light emitting layer, hereafter denoted by the term “organic interface”) is a field which is not yet resolved, and is considered to be a vital point in order to reach the low driver voltage of a single layer structure.
In addition, the other problem caused by the energy barrier is considered to be an influence on the element lifetime of the organic light emitting element. Namely, brightness drops because the carrier movement is impeded and charges accumulate.
No definite theory has been established regarding the mechanism of this degradation. However, it has been reported that the drop in brightness can be suppressed by inserting a hole injecting layer between the anode and the hole transporting layer, and in addition, by performing ac drive at a short wavelength instead of dc drive (Reference 6: S. A. VanSlyke, C. H. Chen, and C. W. Tang, “Organic electroluminescent devices with improved stability,” Applied Physics Letters, Vol. 69, No. 15, 2160-2162 (1996)). This can be said to be experimental evidence that the reduction in brightness can be suppressed in accordance with eliminating charge accumulation by adding the hole injecting layer and by using ac drive.
From the above discussion, the lamination structure has the merits of being able to easily increase the carrier recombination efficiency, and being able to increase the breadth of selection of materials from the vantage of separating functions. However, carrier mobility is suppressed due to the formation of many organic boundaries, and this in turn influences drops in the driver voltage and in brightness.